1. Field of the Invention
The present invention relates generally to optical waveguide apparatus for use with an optical communication, an optical integrated circuit and the like and, more particularly, is directed to a second harmonic generator of optical waveguide type based on Cerenkov radiation.
2. Description of the Prior Art
An optical waveguide in an optical communication, an optical integrated circuit and the like must be made of optical waveguide material having a refractive index as high as possible so that it can fully confine a waveguide light.
As a low loss waveguide, an amorphous film such as Ta.sub.2 O.sub.5 and Nb.sub.2 O.sub.5, a single crystal film of PLZT (Pb-based Lanthanum-doped Zirconate Titanates) and chalcogenide such as As.sub.2 S.sub.5 are provided to make an optical waveguide of high refractive index. The refractive indices of the above-noted constituents are: Ta.sub.2 O.sub.5 has a refractive index of 1.9 to 2.2; Nb.sub.2 O.sub.5 has a refractive index of 2.1 to 2.3; PLZT has a refractive index of about 2.6; and chalcogenide has a refractive index higher than 2.3. However, there are presented the following problems: the PLZT has to be grown as a single crystal film so that as a substrate for this film growing-process, there is used only a single crystal substrate such as sapphire and the like; chalcogenide-based material has high light absorption property in the area of visible light so that it can not make an optical waveguide which is used in the area of visible light; and Ta.sub.2 O.sub.5 has a small refractive index compared with those of LiNbO.sub.3 substrate and LiTaO.sub.3 substrate which are frequently used in the optical IC so that it can not make an optical waveguide for such substrate.
In this manner, a thin film optical waveguide can not be formed on a substrate such as LiNbO.sub.3 and LiTaO.sub.3 so as to achieve low propagation loss (&lt;1dB/cm) over a wide range from a near-infrared ray area to a visible light area.
A second harmonic generator is known, which generates, when supplied with a light having a frequency .omega., a second harmonic wave light of frequency 2.omega.. According to the second harmonic generator, the wavelength region can be enlarged and, with the enlarged wavelength region, a laser apparatus are utilized in a wider variety of applications, and the use of laser light can be optimized in various technical fields. For example, in the optical recording and reproducing process and magneto-optical recording and reproducing process by use of laser light, it becomes possible to increase recording density by reducing the wavelength of laser beam.
Journal of Applied Physics Letters 17, 447 (1970) reported a second harmonic generator of Cerenkov radiation type in which a linear optical waveguide is formed on a non-linear single crystal substrate, and which is supplied with a fundamental wave to achieve a second harmonic wave radiation mode to generate a secondary harmonic wave from the substrate side. This type of second harmonic generator comprises a ZnO non-linear single crystal substrate and a ZnS polycrystal optical waveguide formed thereon and generates a second harmonic wave of 0.53 micrometer by use of a Nd:YAG laser having a wavelength of 1.06 micrometers. In this second harmonic generator, the waveguide thereof is made of polycrystalline material so that its propagation loss is large and a d constant of the ZnO substrate is small, which provides a considerably degenerated efficiency.
Japanese Laid-Open Patent Gazette No. 61-189524 discloses a second harmonic generator. This previously-proposed second harmonic generator employs a LiNbO.sub.3 (hereinafter, simply referred to as LN) substrate which has an optical waveguide constitution by exchanging its proton. In this case, it is to be noted that a semiconductor laser of 0.84 micrometer is utilized as a fundamental light source, and which is supplied with an input of about 100 mW for generating an SHG light of 1 to 2 mW. The most specific feature of the SHG element of this system lies in an automatic phase matching process. In these SHG elements, however, SHG efficiency is less than 1 to 2% relative to the input of 100 mW, and cannot achieve SHG efficiency of, for example, 10% which is required in practice. Further, the proton exchange-process is effective only for the LN substrate, and cannot be applied to other substrates such as LiTaO.sub.3 and KNbO.sub.3.
In the case of the Cerenkov radiation-type second harmonic generator, the efficiency .eta. has a relationship for a d constant of non-linear optical material, fundamental wave power density P.sup..omega. and mutual action length l EQU .eta..alpha.d.sup.2 .multidot.P.sup..omega. .multidot.l
Accordingly, in order to gain a high efficiency of second harmonic generator, a material having a large d constant must be used, the fundamental wave power density P.sup..omega. must be increased and the mutual action length l must be increased. The value of d constant is changed depending on the geometrical relation of its crystal azimuth and polarized wave direction of fundamental wave even in the same material. In the case of LN, the value of d constant is varied as EQU d.sub.31 =-5.9.times.10.sup.-12 (m/V) EQU d.sub.22 =4.0.times.10.sup.-12 (m/V) EQU d.sub.33 =-34.4.times.10.sup.-12 (m/V)
Thus, .vertline.d.sub.33 .vertline. is largest. In the proton exchange-process, only a refractive index n for an extraordinary ray can be increased. Further, in the proton exchange-process, an X plate and a Y plate are etched during the proton exchange-process and the surfaces thereof are made coarse, whereby the LN as the non-linear single crystal substrate can utilize only the Z plate (substrate having a plane perpendicular to z-axis extended along c-axis) and utilize only the TM wave mode. Thus, when the semiconductor laser is employed as the light source, a 1/2 wavelength plate must be provided between the semiconductor laser and the input end of the optical waveguide of the second harmonic generator so as to rotate the polarized wave direction of the laser light by 90 degrees. This requires a collimator lens for introducing a laser light into the 1/2 wavelength plate as a collimated light and an objective lens for converging the light, traveled through the 1/2 wavelength plate, on the optical waveguide of the second harmonic generator, which provides a complicated and large-sized optical system.
Further, a refractive index difference .DELTA.n between the waveguide and the substrate is about 0.14 at maximum. There is then a limit on the confinement of light, i.e. the maximum efficiency. In addition, the proton exchange-process is effective only for the LN and can not be applied to LiTaO.sub.3, KNbO.sub.3 or the like.
Furthermore, in order to increase the SHG efficiency, it becomes important to improve an overlap between the fundamental guide mode and the secondary radiation mode. When the single-poled LiNbO.sub.3 substrate is employed, there is then a limit to improving the overlap between the fundamental guide mode and the secondary radiation mode. Thus, the SHG efficiency can not be improved satisfactorily.
As shown in FIG. 16, the radiated SHG wave has a lateral spread angle of larger than 10 degrees and has a beam spot S of a crescent-shape. There is then presented a problem of a converging characteristic.